Alkaline fluoroborate in fluorophosphate glass free of beryllium

ABSTRACT

Fluorophosphate glass free of beryllium with a refractive index n e  between 1.40 and 1.46, an Abbe value v e  between 85 and 94 and a high positive anomalous partial dispersion value +Δv e  between 19 and 28 is described. The glass is melted from a mixture containing as groups of components metaphosphate, fluorides and an alkali fluoroborate in certain proportions expressed in % by weight, in the following manner: the intermixed mixtue is heated to a temperature between 800° and 800° C., the melt refined at a temperature between 900° and 1000° C. for a period of 8 and 25 minutes, homogenized at a temperature between 900° and 1000° C. within a period of time of 5 to 15 minutes, the temperature reduced to a pouring temperature between 575° and 650° C. and the melt poured into casting molds preheated to approximately 320° C., in particular aluminum molds.

BACKGROUND OF THE INVENTION

The subject matter of the invention is a process for the production of afluorophosphate glass free of beryllium with a refractive index n_(e)between 1.40 and 1.46, an Abbe value v_(e) between 85 and 94 and a highanomalous partial dispersion value +.increment.v_(e) between 19 and 28.

In correcting the so-called "secondary spectrum" of special opticaldevices the computing optician heretofore had been restricted to thesupplemental use of natural or synthetic crystals, for example, themineral fluorite (CaF₂). Because the properties of the material areinadequate for advanced manufacturing methods, for example, completecleavability on the octahedral planes and the relatively low hardness,attempts have been made recently to develop optical glasses with opticaland physical properties similar to those of cubic fluorite, but withbetter chemical and processing properties. This involves not only theparameter constellation of the refractive index n_(e) (fluorite: n_(e)=1.4349) and the Abbe value v_(e) (fluorite: v_(e) =94.8), but alsoadditionally the anomalous partial dispersion value expressed as thedeviation (positive or negative) from the so-called "standard line".increment.v_(e). This value in the case of fluorite amounts to.increment.v_(e) =+31.5.

Fluorophosphate based glasses having additionally positive anomalouspartial dispersion values are already known. They are, however, outsidethe range of the present invention with respect to their n_(e) -v_(e)parameter pairs. Thus, a glass with the values of n_(D) =1.38 and v=95.5is known from CA-P 570 108, a fluoride glass with n_(D) =1.35 and v=81from U.S. Pat. No. 2,716,069, a glass with n_(d) =1.33 and v_(e) =105from GB-P 845 952 and a fluoride glass with n_(D) =1.39 and v_(e) =99.5from U.S. Pat. No. 2,511, 224.

Aside from the fact that the optical positional ranges of these glassesin the n-v diagram differ from those of the glasses produced by theprocess of the invention, the known glasses without exception containberyllium fluoride (BeF₂) as the glass former. Due to the toxicity ofthe beryllium compounds, the use of these components of the mixture maybe harmful to health during the melting process and the subsequentprocessing of the glass.

Even though a fluorine containing phosphate glass free of alkalies andberyllium is known from DE-P 1 088 674--it may also contain smalladditions of B₂ O₃ --the optical properties of the glass with refractiveindices n_(d) between 1.536 and 1.589 and Abbe values v between 66.4 and72.8 are in an entirely different area of the optical diagram. This isalso true for the fluorophosphate glasses known from DE-AS 2 342 484with n_(e) >1.57 and v_(e) <70; for the glasses described in DE-P 1 596877 with n_(e) between 1.47 and 1.53 and v_(e) between 75 and 85 and foran optical glass known from DE-P 1 496 566 with n_(e) values between1.53 and 1.55 and v_(e) values between 62 and 74.

Finally, fluorophosphate glasses have already been described, but thesecontain as a further component, among others, B₂ O₃, for these see U.S.Pat. No. 3,656,976 (n_(e) :1.47 to 1.54; v_(d) : 84.2 to 71.6) and DE-OS2 024 613 (n_(e) : 1.482, v: 84).

The glasses, however, due to the presence of B₂ O₃, have lower Abbevalues and higher refractive indices.

SUMMARY OF THE INVENTION

It is therefore the object of the present invention to provide a processfor the preparation of special optical fluorophosphate glasses that arefree of the disadvantages of the known glasses of a similar type.

The object is attained in a process of the afore-mentioned typeaccording to the invention by that a mixture with the followingcomponents is melted: either 7 to 15% by weight of Al(PO₃)₃ as the solemetaphosphate component, or in the case wherein in place of the aluminummetaphosphate another metaphosphate is used or such a metaphosphate isemployed additionally to the aluminum metaphosphate, 0 to 15% by weightof Al(PO₃)₃ and/or up to 11% by weight of an alkali metaphosphate, inparticular NaPO₃, or respectively, 0 to 15% by weight Al(PO₃)₃ and up to5% by weight of at least one alkaline earth metaphosphate, in particularmagnesium metaphosphate (Mg(PO₃)₂), and/or calcium metaphosphate(Ca(PO₃)₂), wherein the sum of all metaphosphates amounts to between 7and 15% by weight, 0 to 6% by weight lithium fluoride (LiF), 0 to 12% byweight potassium hydrogen fluoride (KHF₂), 3 to 12% by weight MgF₂, 9 to28% by weight calcium fluoride (CaF₂), 6 to 16% by weight strontiumfluoride (SrF₂), 4 to 13% by weight barium fluoride (BaF₂), 22 to 31% byweight aluminum fluoride (AlF₃), wherein the proportion of the alkalineearth fluorides amounts to between 23 and 52% by weight, and 5 to 29% byweight of sodium fluoborate (NaBF₄). Additionally, up to 2% by weight ofat least one of the following supplemental substances may be added tothe mixture; lanthanum fluoride (LaF₃), zinc fluoride (ZnF₂) and/orcadmium fluoride (CdF₂).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

It is of advantage, to melt the above-cited glass mixture in accordancewith the following process: heating the intermixed composition in aplatinum crucible to a temperature between 700° and 800° C.; refiningthe melt at a temperature between 900° and 1000° C. for a time between 8and 25 minutes, homogenizing the melt at a temperature between 900° and1000° C. within a period of time between 5 and 15 minutes, reducing thetemperature to the pouring temperature of between 575° and 650° C.,pouring into molds preheated to approximately 320° C., particularlyaluminum molds.

The introduction of boron ions by means of the complex compound NaBF₄into the melt prevents the formation of B₂ O₃ and thus eliminates thenegative effect of boric acid (boric acid anhydride) on the anomalouspartial dispersion. Furthermore, the fluorophosphate glasses produced bythe process of the invention have a lesser tendency to crystallize andmay be melted in larger units.

Examples of the composition of the mixture according to the inventionare cited in the tables following hereinafter (in % by weight).

                                      TABLE I                                     __________________________________________________________________________    No. of Melt:                                                                          1    2    3    4    5    6    7    8    9    10   11                  __________________________________________________________________________    Al(PO.sub.3).sub.3                                                                    7.5  7.5  10.5 10.5 10.5 --   5.5  5.5  5.5  14.5 14.5                NaPO.sub.3                                                                            --   --   --   --   --   10.5 5.0  --   --   --   --                  Mg(PO.sub.3).sub.2                                                                    --   --   --   --   --   --   --   5.0  --   --   --                  Ca(PO.sub.3).sub.2                                                                    --   --   --   --   --   --   --   --   5.0  --   --                  LiF     4.2  4.2  6.0  4.0  3.0  --   --   --   --   --   --                  KHF.sub.2                                                                             7.2  6.2  8.0  6.0  5.0  --   --   --   --   9.0  12.0                MgF.sub.2                                                                             5.2  5.2  4.0  5.0  5.5  6.0  6.0   6.0 6.0  6.0  6.0                 CaF.sub.2                                                                             11.8 11.8 9.2  11.6 12.8 24.0 24.0 24.0 24.0 24.0 24.0                SrF.sub.2                                                                             7.8  7.8  6.0  7.6  8.4  9.2  9.2  9.2  9.2  9.2  9.2                 BaF.sub.2                                                                             5.7  5.7  4.5  5.5  6.0  6.5  6.5  6.5  6.5  6.5  6.5                 AlF.sub.3                                                                             29.2 30.2 30.8 28.8 27.8 26.8 26.8 22.8 22.8 22.8 22.8                NaBF.sub.4                                                                            21.4 21.4 21.0 21.0 21.0 17.0 17.0 21.0 21.0 8.0  5.0                 n.sub.e 1.4091                                                                             1.4098                                                                             1.4125                                                                             1.4224                                                                             1.4255                                                                             1.4302                                                                             1.4350                                                                             1.4383                                                                             1.4388                                                                             1.4515                                                                             1.4539              v.sub.e 93.71                                                                              93.90                                                                              92.58                                                                              90.99                                                                              90.79                                                                              89.24                                                                              89.76                                                                              88.93                                                                              88.93                                                                              87.17                                                                              86.57               θ'.sub.g                                                                        0.4733                                                                             0.4720                                                                             0.4734                                                                             0.4760                                                                             0.4724                                                                             0.4713                                                                             0.4745                                                                             0.4743                                                                             0.4753                                                                             0.4762                                                                             0.4776              Δv.sub.e                                                                        +27.8                                                                              +27.0                                                                              +26.6                                                                              +26.9                                                                              +24.9                                                                              +21.9                                                                              +24.7                                                                              +23.7                                                                              +24.3                                                                              +23.3                                                                              +23.6               __________________________________________________________________________

                                      TABLE I A                                   __________________________________________________________________________    (MOLE %)                                                                      No. of Melt:                                                                          1    2    3    4    5    6    7    8    9    10   11                  __________________________________________________________________________    Al(PO.sub.3).sub.3                                                                    18.5 18.6 25.3 24.7 24.5 --   13.7 13.0 13.0 32.3 32.5                NaPO.sub.3                                                                            --   --   --   --   --   11.0 4.8  --   --   --   --                  Mg(PO.sub.3).sub.2                                                                    --   --   --   --   --   --   --   8.2  --   --   --                  Ca(PO.sub.3).sub.2                                                                    --   --   --   --   --   --   --   --   8.8  --   --                  LiF     1.0  1.0  1.4  0.9  0.7  --   --   --   --   --   --                  KHF.sub.2                                                                             5.3  4.5  5.7  4.2  3.5  --   --   --   --   5.9  7.9                 MgF.sub.2                                                                             3.0  3.0  2.3  2.8  3.0  3.8  3.5  3.4  3.3  3.1  3.2                 CaF.sub.2                                                                             8.6  8.6  6.6  8.1  8.8  19.3 17.6 16.8 16.7 15.8 15.9                SrF.sub.2                                                                             9.2  9.2  6.9  8.5  9.3  11.9 10.9 10.4 10.3 9.8  9.8                 BaF.sub.2                                                                             9.4  9.4  7.2  8.6  9.3  11.7 10.7 10.3 10.2 9.6  9.7                 AlF.sub.3                                                                             23.0 23.7 23.6 21.6 20.6 23.1 21.2 17.2 17.1 16.1 16.3                NaBF.sub.4                                                                            22.0 22.0 21.0 20.6 20.3 19.2 17.6 20.7 20.6 7.4  4.7                 n.sub.e 1.4091                                                                             1.4098                                                                             1.4125                                                                             1.4225                                                                             1.4255                                                                             1.4302                                                                             1.4350                                                                             1.4383                                                                             1.4388                                                                             1.4515                                                                             1.4539              v.sub.e 93.71                                                                              93.90                                                                              92.58                                                                              90.99                                                                              90.79                                                                              89.24                                                                              89.76                                                                              88.93                                                                              88.93                                                                              87.17                                                                              86.57               θ'.sub.g                                                                        0.4733                                                                             0.4720                                                                             0.4734                                                                             0.4760                                                                             0.4724                                                                             0.4713                                                                             0.4745                                                                             0.4743                                                                             0.4753                                                                             0.4762                                                                             0.4776              Δv.sub.e                                                                        +27.8                                                                              +27.0                                                                              +26.6                                                                              +26.9                                                                              +24.9                                                                              +21.9                                                                              +24.7                                                                              +23.7                                                                              +24.3                                                                              +23.3                                                                              +23.6               __________________________________________________________________________

                                      TABLE II                                    __________________________________________________________________________    No. of Melt:                                                                         12   13   14   15   16   17   18   19   20                             __________________________________________________________________________    Al(PO.sub.3).sub.3                                                                   8.5  8.5  8.5  10.5 14.5 10.5 12.5 14.5 14.5                           MgF.sub.2                                                                            12.0 6.0  6.0  6.0  3.9  6.0  6.0  5.1  5.7                            CaF.sub.2                                                                            24.0 24.0 24.0 28.0 17.0 24.0 24.0 21.0 23.0                           SrF.sub.2                                                                            9.2  15.2 9.2  9.2  6.4  9.2  9.2  8.0  8.8                            BaF.sub.2                                                                            6.5  6.5  12.5 6.5  4.4  10.5 8.5  5.6  6.2                            AlF.sub.3                                                                            22.8 22.8 22.8 22.8 24.8 22.8 22.8 24.8 24.8                           NaBF.sub.4                                                                           17.0 17.0 17.0 17.0 29.0 17.0 17.0 21.0 17.0                           n.sub.e                                                                              1.4321                                                                             1.4375                                                                             1.4412                                                                             1.4435                                                                             1.4451                                                                             1.4452                                                                             1.4505                                                                             1.4553                                                                             1.4559                         v.sub.e                                                                              90.24                                                                              90.53                                                                              89.89                                                                              88.89                                                                              86.70                                                                              88.60                                                                              87.62                                                                              85.75                                                                              86.56                          θ'.sub.g                                                                       0.4725                                                                             0.4767                                                                             0.4774                                                                             0.4752                                                                             0.4739                                                                             0.4746                                                                             0.4755                                                                             0.4727                                                                             0.4750                         Δv.sub.e                                                                       +23.7                                                                              +27.0                                                                              +26.8                                                                              +24.3                                                                              +21.2                                                                              +23.6                                                                              +23.2                                                                              +19.3                                                                              +21.8                          __________________________________________________________________________

                                      TABLE II A                                  __________________________________________________________________________    (MOLE %)                                                                      No. of Melt:                                                                         12   13   14   15   16   17   18   19   20                             __________________________________________________________________________    Al(PO.sub.3).sub.3                                                                   20.5 19.8 19.3 23.5 31.3 23.5 27.6 31.5 31.6                           MgF.sub.2                                                                            6.8  3.3  3.2  3.2  2.0  3.2  3.1  2.6  2.9                            CaF.sub.2                                                                            17.1 16.5 16.1 18.5 10.8 15.9 15.6 13.5 14.8                           SrF.sub.2                                                                            10.6 16.9 9.9  13.2 6.6  9.8  9.6  8.3  9.1                            BaF.sub.2                                                                            10.4 10.1 18.9 9.6  6.3  15.6 12.5 8.1  9.0                            AlF.sub.3                                                                            17.5 16.9 16.5 16.2 17.0 16.2 16.0 17.1 17.2                           NaBF.sub.4                                                                           17.1 16.5 16.1 15.8 26.0 15.8 15.6 18.9 15.4                           n.sub.e                                                                              1.4321                                                                             1.4375                                                                             1.4412                                                                             1.4435                                                                             1.4451                                                                             1.4452                                                                             1.4505                                                                             1.4553                                                                             1.4559                         v.sub.e                                                                              90.24                                                                              90.53                                                                              89.89                                                                              88.89                                                                              86.70                                                                              88.60                                                                              87.62                                                                              85.75                                                                              86.56                          θ'.sub.g                                                                       0.4725                                                                             0.4767                                                                             0.4774                                                                             0.4752                                                                             0.4739                                                                             0.4746                                                                             0.4755                                                                             0.4727                                                                             0.4750                         Δv.sub.e                                                                       +23.7                                                                              +27.0                                                                              +26.8                                                                              +24.3                                                                              +21.2                                                                              +23.6                                                                              +23.2                                                                              +19.3                                                                              +21.8                          __________________________________________________________________________

In the tables, initially the metaphosphate compounds are listed ingroups. The most important component of this group is Al(PO₃)₃, which isabsent in Example 6 only and has been replaced by an equivalent amountof NaPO₃. Examples 7 to 9 contain Al(PO₃)₃ and additionally a furthermetaphosphate, i.e. NaPO₃ (Example 7), Mg(PO₃)₂ (Example 8) and Ca(PO₃)₂(Example 9). By means of these partial exchanges within themetaphosphate component group, planned fine adjustments of the opticalparameters (n_(e), v_(e), +.increment.v_(e)) may be effected.

The second group includes all "simple" fluorides, i.e., the fluorides ofmetals with I, II and III valences and hydrogen fluoride. While LiF andKHF₂ are used selectively--see Examples 6 to 9 wherein monovalent,simple fluorides are entirely absent--the alkaline earth fluorides ofMg, Ca, Sr and Ba are essential components of this mixture group; theyare present simultaneously. This is also true for AlF₃, which is to beadded to the melting charge in relatively constant amounts. Here again,glass chemists are able to adjust the desired optical values by means ofthe planned selection of individual components.

The third essential compound is a complex sodium fluoborate that may beadded within relatively broad limits. The fluoborates are salts offluoboric acid and may be prepared by the dissolution of thecorresponding metal oxides, hydroxides or metal carbonates in aqueousfluoboric acid (HBF₄), see also: HOLLEMAN-WIBERG "Textbook of InorganicChemistry", Edition 40-46, Walter de Gruyter & Co., Berlin, 1958, pages372 and 373, and are to be written in the manner of the chemistry ofcomplexes as: Me^(x) [BF₄ ]_(x), or as a "complex formula" (see:HOLLEMAN-WIBERG, as cited hereinabove, page 159): ##STR1## wherein Mesignifies "metal" and x the valence of the metal (x=I, II, . . . ).

Finally, in the first vertical column of the tables the opticalparameters of the glasses obtained by the process according to theinvention are given, i.e.,

n_(e) =refractive index;

v_(e) =Abbe number (reciprocal value of dispersion);

θ_(g) =(true) anomalous partial dispersion, wherein ##EQU1## wherein thelower indices have the following significance: g=blue mercury line(435.84 nm),

F'=blue cadmium line (479.99 nm),

C'=red cadmium line (643.99 nm),

+.increment.v_(e) =positive deviation from the "standard straight line",as represented graphically in the drawing of DE-PS 1 496 563. Thisdifference value is being designated in the present description and inthe claim--as is generally customary--as the positive anomalous partialdispersion value.

The process for the preparation of the beryllium-free, boron-containingfluorophosphate glasses shall be explained in more detail hereinafterwith the aid of the following example of a melt:

A weighed charge of 2 kg, consisting of

10.5% by weight Al(PO₃)₃

4.0% by weight LiF

6.0% by weight KHF₂

5.0% by weight MgF₂

11.6% by weight CaF₂

7.6% by weight SrF₂

5.5% by weight BaF₂

28.8% by weight AlF₃

21.0% by weight NaBF₄,

is placed after intensive mixing in portions in a platinum cruciblepreheated to 750° C. Following the melting of the charge mixture themelt is refined at 950° C. for 10 minutes, homogenized at the sametemperature for another 10 minutes under constant agitation, reduced intemperature to the necessary pouring temperature of 600° C. and pouredin an aluminum mold preheated to 320° C.

The optical parameters of the glass melted in keeping with theabove-described process are as follows:

n_(e) =1.4225

v_(e) =90.99

g=0.4760

.increment.v_(e) =+26.9

The individual time and temperature intervals to be observed during theentire melting process naturally vary as a function of the actualweighed charge. For the formula given, however, they are within thefollowing ranges: the temperature range of the preheated platinumcrucible is between 700° and 800° C.; the melt is then refined at atemperature between 900° and 1000° C. within a period of time of 8 to 25minutes; then homogenized within the same temperature range during aperiod of 5 and 15 minutes with constant agitation, susequently reducedin temperature to a pouring temperature of 575° to 650° C. and finallypoured into a casting mold preheated to approximately 320° C.

As is well known in the technology of glass, it is within the scope ofthe present invention to add supplemental charges, such as LaF₃, ZnF₂and/or CdF₂, which have favorable effects on the melting process in achemical technical sense.

The examples shown in the tables are to be considered representativeexamples of embodiment. Beyond them, all of the % by weight valuescovered by the ranges specified in the principal claim for thecomponents listed and for compounds, both anions and cations, that actchemically in a similar manner, may be applied in order to achieve theintended result.

What is claimed is:
 1. A fluorophosphate glass free of beryllium andhaving a refractive index n_(e) between 1.40 and 1.46, an Abbe valuev_(e) between 85 and 94 and a high positive anomalous partial dispersion+.increment.v_(e) between 19 and 28 produced by a process comprisingmelting a mixture consisting essentially of the following components:(a)methaphosphate(s): about 7 to 15% by weight Al(PO₃)₃ alone, or about 0to 15% by weight Al(PO₃)₃ and 0 to 11% by weight of an alkalimetaphosphate, or about 0 to 15% by weight (Al(PO₃)₃ and about 0 to 5%by weight of at least one alkaline earth methaphosphate, wherein the sumof all metaphosphates is between about 7 and 15% by weight; (b)fluorides: about 0 to 6% by weight LiF about 0 to 12% by weight KHF₂about 3 to 12% by weight MgF₂ about 9 to 28% by weight CaF₂ about 6 to16% by weight SrF₂ about 4 to 13% by weight BaF₂ about 22 to 31% byweight AlF₃ wherein the proportion of the alkaline earth fluorideamounts to between about 23 and 52% by weight; and (c) alkalinefluoborate: about 5 to 29% by weight NaBF₄, wherein boron ions areintroduced by means of the complex fluoborate to prevent formation of B₂O₃.
 2. A fluorophosphate glass as defined in claim 1, wherein saidmixture further comprises from about 0 to 2% by weight of at least onesubstance selected from the group comprising LaF₃, ZnF₂ or CdF₂.
 3. Afluorophosphate glass as defined in claim 1, wherein said alkalimetaphosphate comprises sodium metaphosphate NaPO₃, and said alkalineearth metaphosphate comprises magnesium metaphosphate Mg(PO₃)₂ orcalcium metaphosphate Ca(PO₃)₂.
 4. A fluorophosphate glass as defined inclaim 1, wherein the glass is produced by the process stepscomprising:(a) heating the intermixed mixture to a temperature between700° and 800° C.; (b) refining the melt at a temperature between 900°and 1000° C. for a period of time between 8 and 25 minutes; (c)homogenizing the melt at a temperature between 900° and 1000° C. withina period of time between 5 and 15 mintues; (d) reducing the temperatureto a pouring temperature between 575° and 650° C.; and (e) pouring intoa casting mold preheated to approximately 320° C.
 5. A fluorophosphateglass as defined in claim 1, wherein the mixture contains more than 17%by weight of NaBF₄.